In recent years, with the development of portable devices such as personal computers and mobile phones, there has been rapidly increasing demand for batteries usable as a power source thereof. In cells of the batteries for the purposes, a liquid electrolyte (an electrolytic solution) containing a combustible organic diluent solvent has been used as an ion transfer medium.
The cell using such an electrolytic solution may cause problems of solution leakage, explosion, etc.
In view of solving the problems, all-solid-state cells, which use a solid electrolyte instead of the liquid electrolyte and contain only solid components to ensure intrinsic safety, have been developing. The all-solid-state cell contains a sintered ceramic as the solid electrolyte, and thereby does not cause the problems of ignition and liquid leakage, and is hardly deteriorated in battery performance by corrosion. Particularly all-solid-state secondary lithium cells can achieve a high energy density easily, and thus have been actively studied in various fields (see, for example, Japanese Laid-Open Patent Publication Nos. 2000-311710 and 2005-063958).
As described above, the all-solid-state cell is excellent in safety and so on. However, since the all-solid-state cell contains only the solid components including the solid electrolyte, it faces major problems in terms of increasing the ion conductivity of the solid electrolyte, reducing the connection resistance between electrolyte particles (the particle boundary resistance), reducing the charge transfer resistance at the connection interface between the electrolyte and an electrode, etc.
For example, in a conventional liquid-type secondary lithium ion cell using the liquid electrolyte, the electrolyte penetrates between particles of a solid electrode, and thus the connection area between the electrolyte and the solid electrode corresponds not to the plane surface area but to the specific surface area of the solid electrode. Further, when the electrolyte sufficiently penetrates in the solid electrode, the connection between the electrolyte and the solid electrode can be maintained in the desired state.
In contrast, the all-solid-state cell is generally produced by applying an electrode material (such as an active material precursor) to a plane surface of the solid electrolyte and by firing the resultant to form an electrode. In this production, the connection area between the solid electrolyte and the electrode does not exceed the plane area of the electrode. Practically, the connection area is the total of contact areas between particles of the solid electrolyte and the electrode, and thereby is generally smaller than the surface area of the electrode.
For increasing the contact area between the solid electrolyte and the electrode active material, and thereby lowering the charge transfer resistance therebetween, for example, an all-solid-state secondary lithium cell having a composite electrode formed by filling pores of a porous solid electrolyte with an electrode active material is disclosed in Japanese Laid-Open Patent Publication No. 2006-260887, etc.
However, no specific process for filling the pores of the porous solid electrolyte with the electrode active material is described at all in Japanese Laid-Open Patent Publication No. 2006-260887. There has been no known suitable method for filling the pores with a larger amount of the electrode active material.